1. Field of the Invention
The present invention relates to a wavelength tunable filter for use in a wavelength division multiplexing transmission system and apparatus in optical communications, and also relates to a wavelength tunable filter device using the same.
2. Related Art of the Invention
In optical communications, there has been known a technique of wavelength division multiplexing which transmits signals of multiple channels through a single optical fiber. In wavelength division multiplexing, signals of different channels are optically multiplexed at different wavelengths by using a optical multiplexer at the transmitting end, and the thus multiplexed signals are transmitted through a single fiber to the receiving end; at the receiving end, the multiplexed signals are separated according to their wavelengths by means of a wavelength filter, to recover the original signals. In particular, by using a wavelength tunable filter whose wavelength is capable of being varied for signal reception, the light of desired wavelength can be separated from the light on which many wavelengths are multiplexed.
There are various types of wavelength tunable filter; among others, a wavelength tunable filter using a diffraction grating has been studied for applications in wavelength division multiplexing transmission because of its high wavelength selectivity providing good isolation between adjacent wavelengths. In high density wavelength division multiplexing transmission where signals are transmitted at closely spaced wavelengths, enhancing the operational stability and the accuracy in setting the filter for the selected wavelength has been a major challenge in the design of the filter. Since the wavelength selection is performed by rotating the diffraction grating, a minimum angular resolution of one hundred-thousandth revolution (360 degrees/100,000), or 0.05 nm in terms of wavelength, is required of the rotating mechanism if the wavelength selection is to be accomplished for multiplexed light of wavelengths spaced apart on the order of subnanometers. However, such a high resolution characteristic has been unachievable with a conventionally used stepping motor alone, and improvements in the rotating mechanism have been necessary.
FIG. 4 shows the configuration of a prior art wavelength tunable filter device. In FIG. 4(a), reference numeral 105 is a diffraction grating, 501 is a mounting jig, 2 is a lens, 3 is an input fiber, 4. is an output, fiber, 8 is an optical-to-electrical converter, 9 is an intensity level detector, 100 is a rotational position calculator, 130 is a rotational position storage, 140 is a rotational position detector, 120 is a motor driver, 61 is a stepping motor, 601 is a reduction gear, 71 is an encoder, and 801 and 802 are joints. FIG. 4(b) is a diagram showing the mounting jig 501 with the diffracting grating 105 mounted thereon, as viewed from the top of FIG. 4(a).
Wavelength multiplexed light from the input fiber 3 enters the diffraction grating 105 through the lens 2 and is chromatically dispersed so that light of desired wavelength is coupled into the output fiber 4 though the lens 2. The light is then converted by the optical-to-electrical converter 8 into an electrical signal. The mounting jig 501 with the diffraction grating 105 mounted thereon is secured to the shaft of the reduction gear 601 which is connected to the stepping motor 61 via the joint 801. The wavelength of the light to be coupled into the output fiber 4 can be selected by rotating the mounting jig 501. The speed at which the mount jig 501 rotates about its axis is slower than the speed at which the stepping motor 61 rotates about its axis, which serves to enhance the apparent rotational angular resolution.
When varying the wavelength to be selected, the motor driver 120 supplies a drive current to the stepping motor 61 which thus starts to rotate. The electrical signal from the optical-to-electrical converter 8 is input to the intensity level detector 9 for detection of the received light level. A signal from the encoder 71 connected to the stepping motor 61 by the joint 802 is extracted as rotational position information by the rotational position detector 140. While monitoring the rotational position information from the rotational position detector 140 and the received light level from the level detector 9, the rotational position calculator 100 compares them with the rotational position corresponding to the desired wavelength prestored in the rotational position storage 130 and, when the desired rotational position is reached, issues a stop command to the motor driver 120, whereupon the stepping motor 61 stops and the reduction gear 601 also stops.
In the above prior art wavelength tunable filter device, however, the rotational position of the diffraction grating is controlled via the reduction gear. Accordingly, the hysteresis that the reduction gear has in its rotational direction becomes a factor that limits the rotational angular resolution of the diffraction grating. Furthermore, the rotational resolution of the stepping motor is greatly affected by the connecting condition of the joint between the reduction gear and the stepping motor. Moreover, the use of the reduction gear increases not only the overall size but also the axial length of the rotating mechanism, as a result of which stability cannot be secured for the rotational operation of the mounting jig which has eccentricity as shown in FIG. 4(b).
Furthermore, since the rotation control is such that the rotational motion is stopped abruptly at the desired angular (wavelength) position, the absolute angular precision of the stopping motion may degrade.
The present invention has been devised to overcome the problems encountered with the above prior art wavelength tunable filter device, and an object of the invention is to provide a wavelength tunable filter device capable of achieving precise wavelength selection for multiplexed light of wavelengths spaced apart on the order of subnanometers.
The present invention of the first invention is a wavelength tunable filter device comprising: at least two optical fibers through which an optical signal is input or output; a wavelength selective element which said optical signal is input to and output from through a lens and which selects a wavelength; a mounting jig to which said wavelength selective element is fixedly secured; a rotating mechanism, comprising an ultrasonic motor and an encoder, for rotating said wavelength selective element; and a motor controller for controlling said ultrasonic motor for driving, wherein said mounting jig is rigidly mounted directly to a rotating shaft of said ultrasonic motor, and the amount of power that does not exceed the driving power necessary to. cause said ultrasonic motor to start rotating from a stopped condition is intermittently applied to said motor controller.